To obtain acceptable ellipsometer and/or polarimeter performance over a wide spectral range, compensator-based ellipsometer and/or polarimeter designs require a compensator element that provides retardance within a certain acceptable range over the entire spectral range. Traditionally, birefringent waveplates of quartz or MgF2 have been used as compensator elements in rotating element designs. A single waveplate exhibits a (1/wavelength) dependence in retardance vs. wavelength, while a dual/multiple waveplate design, (as disclosed in U.S. Pat. No. 6,353,477), can minimize the effect of the (1/wavelength) dependence.
With the present invention in mind, known relevant patents are:                U.S. Pat. No. 5,706,212 to Thompson et al.;        U.S. Pat. No. 6,353,477 to Johs et al.;        U.S. Pat. No. 5,963,325 to Johs et al.;        U.S. Pat. No. 6,141,102 to Johs et al.;        U.S. Pat. No. 6,084,675 to Herzinger et al.; U.S. Pat. No. 6,118,537 to Johs et al.;        U.S. Pat. No. 6,100,981 to Johs et al.;        U.S. Pat. No. 6,084,674 to Johs et al.        
An application which published Jun. 28, 2007 was recently identified and is US 2007/0146706 A1.
Need remains for additional retarder systems which provide improved characteristics.